Fig. 1. Experimental setup. λ/2, half-wave plate; TFP, thin-film polarizer; M1, plano mirror at 1030 nm; F1−F4, lenses; SHG, 3 mm LBO crystal; CP, compensation plate; DW, dual-wavelength waveplate; SFG, 3 mm BBO crystal; DM, dichroic mirror; M2, plano mirror at 345 nm.

Fig. 2. (a) Spectrum and (b) M² factors of Yb-doped femtosecond fiber laser; the inset in (b) is the near-field beam profile.

Fig. 3. (a) Average output power and conversion efficiency of the SH beam as functions of the fundamental power. Inset, the near-field beam profile of the SH beam at maximum average power output. (b) Spectrum, (c) Mx2, and (d) My2 factors of the SH beam.

Fig. 4. (a) Spatial and temporal walk-off effects and their optimization using a CP. Red, fundamental pulses; green, SH pulses; violet, UV pulses. (b) Group refractive index curves of SH (blue curves) and fundamental (red curves) beams including o (solid line) and e (dashed line) waves in the optical x–z plane of BBO. (c) Delay time and walk-off distance of a 1-mm-thick BBO crystal as functions of θ. Negative value of Δt means that the fundamental pulses lag behind the SH pulses.

Fig. 5. Experimental results of SFG. (a) UV output power for different CPs with different focusing lenses. (b) UV output power and conversion efficiency as functions of the fundamental power. Inset, near-field beam profile of the UV beam at maximum average output power. (c) UV spectrum at maximum average output power. (d) UV output power for different CPs with f₂ = f₃=125mm.

Fig. 6. (a) UV output power and conversion efficiency as functions of the fundamental power using the conventional delay line. Inset, near-field beam profile of the UV beam at maximum average output power. (b) Power stability tests of the CP and conventional delay line, respectively, for 6 h.